: There is an urgent need for disease-modifying treatment of Alzheimer's disease (AD) starting at its very onset. This knowledge gap remains because conventional approaches cannot measure in vivo brain region-specific biomarkers of the earliest relevant dysfunction underlying abnormal behavior. Often, spatial disorientation is observed during prodromal AD, and its occurrence predicts later dementia. A brain region contributing to this spatial confusion is the CA1 subfield of hippocampus because of its essential role in encoding spatial information. HC oxidative stress is most commonly identified at the very start of AD, and in experimental models of AD. Yet, it has not been possible to prove that prodromal oxidative stress in the relevant CA1 subfield plays a pathogenic role in at-risk patients showing impaired spatial memory because conventional methods only measure oxidative stress from post-mortem tissue. Addressing this major knowledge gap requires a new paradigm that compares antioxidant treatment efficacy in HC CA1 subregions in vivo with improved spatial learning and memory in experimental models, and that can then be translated into patients. In this proposal, we present a transformative solution to this problem based on a novel method recently discovered by our lab: QUEnch-assiSTed MRI (QUEST MRI). QUEST MRI is a robust and sensitive tool that has been validated against ?gold standard? methods and maps in vivo excessive free radical production in, for example, murine dorsal CA1. The QUEST MRI index of abnormally high production of paramagnetic free radicals in specific brain regions is a greater- than-normal spin-lattice relaxation rate R1 (1/T1) that can be returned to baseline after acute antioxidant administration. Our QUEST MRI studies have confirmed dorsal HC CA1-specific oxidative stress in spontaneous and familial AD mouse models with declines in spatial learning and memory in conjunction with HC CA1 oxidative stress measured ex vivo. We also find downstream consequences of oxidative stress such as greater-than-normal amounts of the lipid peroxidation product 4-hydroxynonenal (HNE), dorsal HC CA1 calcium dysregulation and reductions in dorsal HC CA1 calcium-dependent afterhyperpolarization (AHP). To improve statistical power, this proposal is tightly focused on uniquely testing a specific working hypothesis that oxidative stress in dorsal CA1 in vivo causes deterioration of spatial memory in experimental models. Our highly innovative studies by an experienced team of experts will validate a new bridging tool for testing in vivo antioxidant therapeutic strategies to mitigate a clinically important early decline in spatial memory preceding later loss of personhood in AD.